Never heard this before.
I’d like to know if it is really true or just anecdotal.
It’s true. The history books were scrubbed of these facts long ago. The holoder of the most slaves in Lousiana was also a free negro, who had the largest sugarcane operation in the state.
The vast majority of the “black slaveholders” were actually mixed-race creoles in Louisiana. The French influence made slaveowners much more likely to acknowledge their children by their slaves, free them and allow them to inherit. They may have generations away from slavery and full-blooded blackness—quadroons, octaroons, etc.—but under the law they were still considered black. So you could quite easily have a “black slaveowner” who was only 1/16th black.